1. Field of the Invention
The present invention relates to a dielectric filter for use in the bands of microwaves, millimeter waves, etc., as well as a transmitting/receiving duplexer and a communication apparatus each using the dielectric filter.
2. Description of the Related Art
Responding to a demand for high-capacity and high-speed communication systems, it has been hitherto planed to extend the frequency band used for communications from the microwave band to the millimeter-wave band. In particular, using the sub-millimeter-wave band in various systems, such as wireless LAN, portable TV phones and next-generation satellite broadcasting, has been taken into consideration. Correspondingly, there has been a demand for filters which have a reduced size, are more inexpensive, and are superior in mounting onto planar circuits. With the above situation in mind, the inventors of this application proposed "Sub-Millimeter-Wave Band-Pass Filter Using Dielectric Resonator of Planar Circuit Type", C-121, General Meeting of The Institute of Electronics, Information and Communication Engineers, 1996.
The structure of the proposed dielectric filter is shown in an exploded perspective view of FIG. 8. In FIG. 8, denoted by reference numeral 3 is a dielectric plate which has electrodes formed on both principal planes thereof with circular electrode non-formed portions of predetermined size defined in the electrodes in opposing relation. Numeral 1 in the drawing denotes the electrode formed on an upper surface of the dielectric plate 3, and 4a, 4b denote the electrode non-formed portions. Denoted by 6 is a base plate and 7 is a frame, these members being each made of a ceramic with .epsilon.r=7.3. Electrodes are formed on a lower surface of the base plate 6, a portion of an upper surface of the base plate 6 extending out of the frame 7, and peripheral portions of the frame 7, thus constituting a lower case. Denoted by 8 is a cover made of a ceramic with .epsilon.r=7.3. The cover 8 has electrodes formed on its surface held in contact with the electrode 1 and on its peripheral surfaces. Formed on the upper surface of the base plate 6 are microstrip lines, serving as input/-output terminals, one of which is shown at 9. Probes 19, 20 are connected respectively to the microstrip lines.
With the above construction, parts or areas of the dielectric plate 3 positioned between the electrode non-formed portions on both the principal planes function as dielectric resonators in the TE010 mode. The dielectric resonators adjacent to each other are electromagnetically coupled with not only each other but also the probes 19, 20, respectively.
In the conventional dielectric filter having the structure explained above, a waveguide path is constituted in an area where the electrodes are formed on both principal planes of the base plate 6. Accordingly, the waveguide path is coupled with the microstrip lines, causing a signal to propagate inside the base plate 6 in the so-called parallel plate mode. This has raised a fear that the attenuation characteristic and the spurious characteristic of the filter may deteriorate.
For that reason, the conventional dielectric filter has been designed to cut off the coupling between the waveguide path constituted by the electrodes on both principal planes of the base plate 6 and the microstrip lines by forming through holes 13 to make electrical conduction between the electrodes on both principal planes of the base plate 6 in the vicinity of the microstrip line 9, as shown in FIG. 8. However, such a design has been not sufficient in some cases to satisfy specific demanded characteristics. Further, because it is not easy to bore highly accurate holes in the ceramic substrate, the above design pushes up a manufacture cost when ceramics are used as materials of base plates on which the microstrip lines are provided. For the base plate 6 having a high specific inductive capacity, the wavelength of a signal propagating through the waveguide path inside the base plate becomes short. This means that when forming a plurality of through holes in array, it is necessary to set small the array pitch and increase the number of the through holes. Additionally, when the base plate is made of a ceramic, the plate thickness is so thin on the order of 0.2-0.5 mm that the base plate is less convenient in handling.